I think you're on the right track when you suspect that something like what you describe as a "frequency spike" may well be at the heart of the issue in question.
Let me assure you that the actual signal path from capsule output to mixer input is as linear as a cable when using XD-V70s and V75s, with no inherent "eq" or companding that could affect the subjective balance of your mix.
I make these comments from the perspective of a working Audio Director/Sound Supervisor who's approaching half a century of ongoing mixing and recording experience, most of it live or "as live" ...and one who has neither affiliation with, nor particular loyalty to, Line 6. (Click on my pink avatar for more details.)
My feeling is that you should experiment with Line 6's different mic models before going to the trouble and expense of capsule replacement.
To my ears, the "standard L6" model does exhibit the open and extended high frequency response that (while being much loved by regular users of condenser vocal mics such as Neumann KMS104s and 105s) could well be giving rise to the "out front" effect that you mention.
I'd be very surprised if you could not find a sound that "sits right" for you and would suggest that you start with the silky bass that Line 6 have modeled from Electro-Voice's neodymium-capsuled N/D767a in their "767" setting.
Then have a listen to the "b58", "935" and "d5" models.
I can report that the "b58" has proven to be the most "familiar" to vocalists in just over a year on the road using my V70 handhelds, with many subsequently relishing the ability to "tailor" my handhelds to their particular vocal styles.
Sir thank you for the quick response. Unfortunately it doesn't seem to matter which mic model I use I still get that same "out front" effect. I think the comment you gave with the condenser mic is pretty accurate it seems to sound more like a condenser (picking up every little detail) vs a dynamic mic (which seems to sound well "muddy" for a lack of a better word lol). I guess the best way to explain it, at least to my ears, it seems like the standard shures and senns seem more "round" and "dark" and therefore sit better in the mix. It seems like maybe there is some bass response lost on the line 6 mic? I've panned through all the models and they all exhibit this same too much out front effect so it appears. I did compare the beta 58 modeling vs a true beta 58 and I could hear a difference. Sure they sound similar as far as eq response but what I'm describing is a loss of "roundness" and the sound is "darker" to my ears for the wired mics. This was literally pulling the plug out of my wireless box and hooking up directly to the beta 58 so it was an apples to apples comparison. Any ideas? Do you think a capsule swap might be the answer? Thanks in advance!
From what you've just noted, Leno, I'm now guessing that we're talking about vocalists who like their mic to sit pretty much against their lips, as that is the only time my old ears discern a difference between the cabled "originals" and Line 6's models that are drawn from them...
And, yes, I feel that, given placement at such a minimal distance, the "proximity effect" definitely yields a slightly more "dark" or "warm" bass from the originals than it does from the Line 6 equivalent. The Line 6 models' accuracy is only restored when the lips are moved further from the capsules by as little as 5mm (1/4 inch).
A further "yes"...
If you're a beta58 user, I'm sure (or, indeed, "Shure") that you'll be absolutely thrilled with the way this capsule http://www.bhphotovideo.com/c/product/341232-REG/Shure_RPW118_RPW118_Dynamic_Replacement_Element.html simply screws on to the XD-V70/V75 handheld, at which point the Line 6 wireless will become absolutely indistinguishable from the cabled version, ...yielding much better results and equivalence, in my opinion, than when it's aboard Shure's own wireless systems.
A fuller discussion of exactly this swap (along with other comments from an ultimately happy "transplantee") can be found here: http://line6.com/support/message/376455#376455
Sadly, the news regarding capsule swaps to other manufacturers is more mixed, but a quick search of this forum will reveal various "work-arounds" and "gain mods" that have ultimately met with success.
It's also regrettable that European manufacturers (other than Telefunken) seem loth to release capsules compatible with the "Shure/Line 6" screw-on "pogo-stick-pin" contact system.
Now we might definitely be on to something! I'm definitely a close to the mic kinda singer. Also I should have mentioned I do use a tc helicon voice live play effects unit, I didn't know if the digital wireless signal some how reacted differently with the effects unit than a standard wired mic but that doesn't make much sense as the signal is processed out through an xlr so I didn't buy that rationale. Although the wireless unit does sometimes act weird with effects like a megaphone effect it will cut in and out a little can't explain that one. However, that proximity effect as you described Ron DOES make sense to me. I will have to try and track down a SM58 capsule locally and try it out and see if it "acts" more like I'm accustomed with a wired Shure maybe that's all it is. I keep hearing good things about that Heil RC35 capsule but don't want to dump the cash on it if it won't change my fundamental problem. Thanks Ron for the useful information!
Bob Heil's legendary 35 happens to be one of my favourites as well, but I'd recommend that you definitely try the cabled version at a music store before committing, ...as its performance differs vastly from a beta58 for most voices.
That being said, it's my firm opinion that the Heil RC35's "cut through" revelation of detail and enhanced immunity from feedback place it several classes above Shure's products.
The good news is that there are quite a few of them being very succesfully used atop XD-V handhelds out there, this discussion being a good indication of what you might expect: http://line6.com/support/message/364749#364749
As you say, I can't imagine that any of TC Helicon's excellent range of stomp boxes and modifiers would have a detrimental effect on the performance of any viable source unless they're really badly maladjusted by some cloth-eared fool.
By the way, should you be considering a trial by using the field-replaceable capsule from a cabled beta58 onto your XD-V handheld, you'll very quickly discover that this won't work. That cabled capsule's a "solder" job, not the "pogo pin" system required for wireless transmitters.